China has urged Malaysia's government to release any information it has regarding the missing Malaysia Airlines jet to help narrow the search area.

The Foreign Ministry's appeal reflected growing frustration among Chinese officials over conflicting information about the plane, which vanished last Saturday with 239 people aboard including 154 Chinese passengers.

The Beijing-bound aircraft last communicated with air traffic base stations east of Malaysia over the South China Sea. The search expanded to the Strait of Malacca, west of Malaysia, after the Malaysian air force said radar showed it might have turned in that direction.

"China urgently appeals to Malaysia for all information they have regarding the search," said Foreign Ministry spokesman Hong Lei. "That will not only help China with its search, but also help all sides in the search to make their search more effective and accurately targeted."

Eight Chinese vessels are taking part in the international search effort. They covered 21,000sq miles of ocean by Thursday evening and were expanding their search area, according to China's official Xinhua News Agency.

Also today, frustrated relatives of missing passengers met in Beijing with Malaysia Airlines' commercial director and pressed for clarification of reports about how long the plane emitted signals while flying.

A US official told The Associated Press that the plane sent signals to a satellite for four hours after it went missing, raising the possibility it could have flown far from the current search areas.

"We don't simply want to get our information from the news media," one man told the executive, Hugh Dunleavy.

Mr Dunleavy said he could not provide any new information, but that he would work on obtaining updates from search officials.